Thanks guys. I’ve conjured up this, which in my mind should work:
function same_youtube_options_controls_acf($content) {
$content = get_sub_field('iframe_url');
return str_replace("rel=0&", "rel0&&autohide=1&showinfo=0&controls=1&", $content);
}
add_filter('acf/load_field/name=iframe_url', 'same_youtube_options_controls_acf');
However, the iframe_url just disappears.
Sorry to be a pain.